1. Field
This disclosure relates to a system and method capable of generating a coherent comb of optical wavelengths. Embodiments described herein are particularly suitable to enable high spectral density communications with carriers spaced at orthogonal frequencies, such as Coherent WDM.
2. Description of Related Technologies
Coherent WDM (Wave Division Multiplexing) are techniques applied to communication systems to achieve high information spectral density (ISD). In Coherent WDM, and optionally in OFDM and its derivatives, the relative optical phase of signals carried on adjacent wavelengths is fixed, such that any interference effects, which impact the channels, are also fixed, and may be beneficially used to enhance the system performance.
Prior art coherent WDM systems typically employ a single mode laser connected to an optical modulator, for example using two or more Mach-Zehnder devices, typically in a cascaded arrangement, to produce a plurality of sidebands or channels, as shown in FIG. 1.
PCT patent publication number WO2007043032, assigned to University College Cork, discloses a transmitter that transmits signals on multiple channels and can be wavelength multiplexed according to Coherent WDM. The quality of the received signals is very much enhanced by the transmitter tuning the relative phases of neighbouring channels according to delay characteristics of the optical path. In one example, the transmitter adds a relative phase shift of π/2 between adjacent carriers and furthermore the resultant beat frequency signal for the odd and even channel groups are time aligned with their respective data signals to counter the receiver side delays introduced by optical and electrical filtering devices. Since the adjacent sub carriers are added interferometrically the relative phases may be set any particular desired value by monitoring either the total power or the phase dependant amplitudes of the beat frequency between them. These effects are discussed in more detail in two papers published by T. Healy, F. C. G. Gunning, A. D. Ellis, “Phase Stabilisation of Coherent WDM Modulator Array”, Proc OFC'06, Paper OTuI5, (2006) and F. C. Garcia Gunning. T. Healy, X. Yang, A. D. Ellis, “0.6 Tbit/s Capacity and 2 bit/s/Hz Spectral Efficiency at 42.6 Gsymbol/s Using a Single DFB Laser with NRZ Coherent WDM and Polarisation Multiplexing”, CLEO Europe 2007, Munich, Germany, paper CI8-5, (2007).
Any harmonics or sub-harmonics will similarly vary with phase and therefore can also be used as an error signal for a phase stabilisation circuit. The phase stabilisation circuit in the transmitter therefore can maintain a desired stable optimum phase alignment between neighbouring channels or sub-bands without the need for feedback from the receiver.
Recent trends in the telecommunication industry demands that telecommunication providers require adequate optical signal to noise ratio's (OSNR) within the transmitter to provide sufficient margin for subsequent transmission. A transmitter OSNR well in excess of 35 dB is needed to ensure less than 0.25 dB penalty for a carrier transmitting a signal at a rate of 40 Gbaud. Values in excess of 45 dB would be more typical. Given the fixed noise levels of optical booster amplifiers, this corresponds to a launch power from an optical sub assembly of 500 μW for each coherent WDM subcarrier. Employing typical modulator based optical comb generators divides the power of a single high power laser between the modes, for example ten modes, and presents a minimum excess insertion loss of 4 dB loss per Mach-Zehnder device. These modes are then separated using filtering devices, with additional excess loss (typically greater than 2 dB). These losses are in addition to the losses associated with the electro-optic modulator necessary to encode the data (typically greater than 4 dB), and losses associated with the recombination of the signals (typically greater than 3 dB). The result of which is, that if a single high power laser is employed, a minimum laser output power in excess of 100 mW is necessary in order to achieve a launch power of 500 μW per channel into a booster amplifier. In practice the total required power is significantly in excess of these values, and so additional optical power amplifiers are used within the transmitter sub assembly, adding to the cost, size, complexity and power consumption of the transmitter. These launch power considerations are enhanced if a booster amplifier is not employed and the transmitter sub assembly is required to drive the communications link directly.
Additional problems are associated with the difficulty of integration of such high power components with high performance Mach-Zehnder modulators, the number of modes generated (e.g. 7, 9, 11, 13) wasted power associated with undesirable side modes and power equalization, the necessity to demultiplex the modes prior to modulation and the excess loss of phase control elements.
A possible solution to avoid the excess loss associated with the comb generating Mach-Zehnder modulators is to use an alternative comb source, such as a mode-locked laser which produces a very broad optical comb as shown in FIG. 2. However a problem with this approach is that it is very difficult to simultaneously obtain the required mode power, compact size, precisely set operating frequency and power distribution. For WDM solutions, the optical comb has far too many undesirable modes, which will need to be filtered out, and this means that the efficiency of the source is very poor. Furthermore, as with the cascaded modulator solution, there is also no way to modulate or select individual lines from within the comb generator without the use of an external optical demultiplexer.
There is currently no demonstrated way to generate the required comb signal required in the telecommunication industry from a compact, or integrated source. Embodiments disclosed herein are directed to providing a solution to the above mentioned problems.